1. Field of the Invention
The present invention relates to a method for forming an oxide superconducting film.
2. Description of the Prior Art
Conventionally, a Nb-Ge metallic material, e.g. Nb.sub.3 Ge, has been utilized as a superconducting material. The coherent length of this material is comparatively long, therefore, it is quite easy to manufacture by using the metallic superconducting material a Josephson device, for example, in which an insulator is provided between superconducting materials. Also, it is actually possible to manufacture a device such as a transistor utilizing proximity effect. Further, since the metallic superconducting material does not have anisotropy, it is not necessary to pay attention to alignment of its axes and the like when manufacturing an electronic device using this metallic material.
However, the metallic superconducting material has a disadvantage that the critical temperature (at which resistance becomes zero) thereof is as low as 23K or less. Therefore, it has been required to manufacture an electronic device utilizing an oxide superconducting material whose critical temperature is higher than the liquid nitrogen temperature (77K), for an industrial application.
Methods for forming a superconducting oxide thin film are divided into two kinds; one is a method in which an oxidation step such as annealing with oxygen is carried out after a film formation step, and the other is a method in which an oxidation such as annealing with oxygen is carried out during film formation. With respect to the former method, a film obtained is a polycrystalline film having weak junctions or Josephson junctions formed on grain boundaries formed in the polycrystalline film. Since oxide superconducting materials essentially have short coherent length, electronic devices such as Josephson devices formed by the former method are affected by these junctions. Therefore, the former method is insufficient for manufacturing devices. On the other hand, the film obtained by the latter method is a single crystalline-like thin film.
For the above reason, the latter method has attracted interests of researchers.
Since superconductivity of an oxide superconducting material is very sensitive to an oxygen amount thereof and the oxide superconducting material exhibits superconductivity only at a fixed amount of oxygen thereof, it is necessary to finely control the oxygen amount. The oxidation of the latter method is conventionally carried out in an atmosphere comprising an oxidizing gas, for example oxygen gas, ozone gas, nitrogen oxide gas, or oxygen plasma, typically oxygen gas. In the case of the oxidation carried out in such an atmosphere, oxidation condition for realizing the fixed oxygen amount is conventionally determined through trial and error. In addition, even if thin films are formed under the same conditions by the latter method, the critical temperatures of the thin films are sometimes substantially different.
For obtaining a single crystalline-like thin film of good quality, high degree of vacuum is preferred during the film formation. The high degree of vacuum is also preferred in order to enlarge mean free path and reduce impurity concentration in the film to be formed. However, in the case that the oxidation of the latter method is carried out in an oxidizing atmosphere such as oxygen atmosphere, the degree of vacuum during the film formation cannot help being lowered. As a result, the quality of the film is badly affected.